1. Field of the Invention
This invention relates to a living subject adaptive blood flow control system and more particularly to an apheresis blood flow control system which optimizes blood flow by limiting the blood flow rate in accordance with a flow control curve determined individually for each donor or patient subject from actual subject data. More particularly, this invention relates to an improved blood flow control system for controlling and optimizing the rate of blood withdrawal from a blood vessel, thereby mitigating the frequency and/or severity of occlusive interruptions (e.g. collapse of vein or collapse of tubing) in the course of blood withdrawal.
2. Discussion of the Prior Art
Blood collection systems and apheresis systems such as plasmapheresis, platelet pheresis, therapeutic plasma exchange or processing, etc. as well as other systems are known which require the extraction or reinfusion of bodily fluids from or to a living subject. In the case of a plasmapheresis system whole blood is extracted from the subject, plasma is separated from the whole blood, and an extraction product containing a higher concentration of blood cells than the whole blood is reinfused back to the subject while the separated plasma is retained and used for desired purposes. Frequently, a selected volume of saline solution or other fluids are infused into the subject to replace the volume of plasma separated from the whole blood.
To optimize utilization of processing equipment and support personnel and minimize inconvenience and discomfort to the subject, it is often desirable to remove bodily fluids as rapidly as possible. However, physiological restrictions on flow rates impose practical limitations on how fast pumping can proceed.
During extraction, if the pumping rate exceeds the flow capacity of a vein into which a phlebotomy needle or catheter is inserted, the intravenous pressure will drop below approximate atmospheric pressure and the vein sidewalls will collapse under atmospheric pressure. In the following, atmospheric pressure refers to the local extravascular pressure surrounding the vein in the location away from the pressure cuff and toward the extremities. When such collapse of the vein occurs, the blood pump must be stopped or significantly slowed until the intravenous blood flow restores the intravenous pressure to a point greater than atmospheric pressure, thus refilling the collapsed portion of the vein.
Oftentimes, when the vein collapses about the needle, the end of the needle will become compressed against the sidewall of the vein. When this happens the needle will frequently become embedded within the vein sidewall or will be sealed to the vein wall by virtue of the negative pressure within the needle and tubing that can be developed following a sudden occlusion. The needle then remains occluded, even after the previously collapsed vein has been refilled with blood. It may then become necessary to remove and reposition the needle at the expense of considerable additional time delay.
Furthermore, whenever the internal vein pressure is allowed to drop below atmospheric pressure and vein collapse occurs, increased fluid flow shear can cause platelet activation or hemolysis. Also, the needle can cause damage to the endothelial cells along the vein wall, leading to blood coagulation. This is particularly undesirable early in the processing, prior to the addition of anticoagulant, since the initiation of coagulation cascade can seriously degrade, or make useless, the desired extracorporeal blood processing.
Predicting the optimal rate at which blood may be extracted from a blood vessel is difficult because intravascular flow rates and volumes vary considerably from subject to subject. Even for a given subject, the intravascular flow rate capacity can vary considerably over a given time period. When blood is being withdrawn from a peripheral vein, (e.g. a superficial vein of the antecubital fossa), moment to moment variations in blood flow through the peripheral vein may be observed due to changes in physiological variables and/or contraction/relaxation of the muscles surrounding the blood vessel. In an effort to maintain relative continuity of blood flow through the vein it is common practice to require the donor to engage in alternate contraction/relaxation of the muscles during the blood withdrawal process--usually by squeezing an object held with the hand adjacent the withdrawal site. If, however, the donor/subject is less than diligent in squeezing the object, or if the donor only squeezes the object for intermittent periods, this may result in extreme variations in blood flow within the peripheral vein during the blood withdrawal process.
Attempting to optimize the pump blood flow rate by sensing flow path pressure adjacent the needle is uncertain because the pressure drop across the needle varies substantially with flow rate, hematocrit dependent blood viscosity and needle size parameters. It is therefore common to rely on a gravity driven flow rate far below the optimum or a pumping rate that is known to be well within the blood flow capacity of most subjects. This may be far below the optimum flow rate.
One arrangement in which a plasmapheresis system serves as a reservoir for receiving and returning bodily fluids is described in U.S. Pat. No. 4,086,924 to Latham, Jr. for "Plasmapheresis Apparatus". In this system extraction occurs under vein pressure and gravity. A multi-rate blood pump for the plasmapheresis system is accelerated or decelerated to match this flow rate. Reinfusion occurs at a predetermined rate with the blood pump set to a relatively low speed condition.
A more capable blood flow control system is disclosed in U.S. Pat. No. 4,657,529 to Prince, et al., which has been assigned to the common assignee herein, and is incorporated herein by reference. As with the present system, the system disclosed in the prior patent utilizes a programmed digital processor to regulate blood flow based on sensed fluid pressure in the flow path. The flow rate, i.e. pump speed, is regulated to achieve a maximum flow rate consistent with avoiding vein occlusions. Though the system disclosed therein provides a significant improvement over prior blood flow control systems, experience has indicated that in some instances, due to errors in the slope measurement for the control curve, it is still possible to create negative pressure in the blood vessel, resulting in vein collapse. Thus, what is needed is an improved system and method for calibrating and controlling fluid withdrawal which utilizes a more accurate slope calculation, thus extending the operating range of the system.